Signal processors and methods for estimating transformations between signals with least squares

ABSTRACT

Signal processing devices and methods estimate transforms between signals using a least squares technique. From a seed set of transform candidates, a direct least squares method applies a seed transform candidate to a reference signal and then measures correlation between the transformed reference signal and a suspect signal. For each candidate, update coordinates of reference signal features are identified in the suspect signal and provided as input to a least squares method to compute an update to the transform candidate. The method iterates so long as the update of the transform provides a better correlation. At the end of the process, the method identifies a transform or set of top transforms based on a further analysis of correlation, as well as other results.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No.14/936,359, filed Nov. 9, 2015 (now U.S. Pat. No. 9,953,390) which is acontinuation of U.S. application Ser. No. 13/224,608, filed Sep. 2, 2011(now U.S. Pat. No. 9,182,778), which claims benefit of Ser. No.61/380,180 filed Sep. 3, 2010.

TECHNICAL FIELD

The invention relates to signal processing, and specifically signalprocessing for determining transformations between signals, for use insignal detection, identification, signal matching and recognitionapplications, among others.

BACKGROUND AND SUMMARY

There are a wide variety of signal processing applications in which theaffine transformation between a suspect signal and a reference signalneed to be computed accurately and efficiently. This is particularly thecase for signal detection and recognition applications for images, andit applies to other types of signals as well. In the case of signaldetection and signal recognition, the objective for the computing deviceis to determine whether a particular reference signal is present in asuspect signal. This objective is more difficult when the referencesignal is present, yet is distorted by a transform of the coordinatespace. In image processing, such transformations are caused bymanipulation of the reference signal through image editing(magnification, shrinking, rotation, digital sampling (and re-sampling),format conversions, etc.). When the reference images or the objects theyrepresent are captured via a camera from a different reference pointrelative to their original state, the result is a suspect image, whichcontains the reference signal, yet in a transformed state. Unless thereis a means to determine and compensate for the affine transformation ofthe reference signal, it is more difficult to accurately detect,recognize or match the reference signal with its counterpart in thesuspect image.

This signal processing problem is important to a variety of fields. Someexamples include machine vision, medical imagery analysis, object andsignal recognition, biometric signal analysis and matching (e.g.,facial, voice, iris/retinal, fingerprint matching), surveillanceapplications, etc. In these applications, the objective may be to detector match an input suspect signal with one particular reference signal,or match it with many different reference signals (such as in databasesearching in which a query includes a suspect signal (a probe ortemplate) that is matched against a reference database of signals).Various types of images and sounds can be identified using signalrecognition and detection techniques. These include recognition based onsignal attributes that are an inherent in signals, as well asrecognition based on signals particularly embedded in another signal toprovide an auxiliary data carrying capacity, as in the case of machinereadable codes like bar codes and digital watermarks.

In recent years, computing devices are becoming increasingly equippedwith sensors of various kinds, including image and audio sensors. Togive these devices the ability to interact with the world around them,they need to be able to recognize and identify signals that they capturethrough the sensors.

The advances of electronics have extended these advanced sensoryfunctions beyond special purpose devices like machine vision equipment,surveillance and exploration equipment, and medical imaging tools, toconsumer electronics devices, like personal computers and mobiletelephone handsets. The signals captured in these devices are oftendistorted by transformations. If these transformations can beapproximated by affine transformations or at least locally affinetransformations, then it may be possible to determine the affinetransformation (including local affine transform in a portion of thesignal) that most closely matches the suspect with a reference signal.

The affine transformation that aligns a reference signal with itscounterpart in a suspect signal can be expressed as y=Ax+b, where x andy are vectors representing the reference and transformed version of thereference signal, A is a linear transform matrix, and b is translation.The affine transformation generally comprises a linear transformation(rotation, scaling or shear) and translation (i.e. shift). The lineartransformation matrix, for two dimensional signals, is a two by twomatrix (2×2) of parameters that define rotation, scale and shear. Thetranslation component is a two by one (2×1) matrix of parameters thatdefine the horizontal and vertical shift. The translation is related tothe phase shift as described in more detail below. Thus, the process ofaligning two signals can include both approximations of the lineartransform as well as the translation. The linear transform is sometimesapproximated by determining signal correlation operations, which oftenemploy Fourier transforms and inverse Fourier transforms. Thetranslation component is approximated by determining phase shift (e.g.,using signal correlation) in a Fourier representation.

When signal transforms are computed in digital computing environments ofgeneral purpose processing units or special purpose digital logiccircuits, a number of challenges arise. Some of these challenges includethe errors caused by representing signals in discrete digital logic. Notonly is quantization error introduced as analog signals are sampledthrough sensors, but also as these signals are re-sampled whentransformed into different coordinate spaces (e.g., Fourier and inverseFourier transforms). Additional errors are introduced in the precisionor limits on precision of the circuitry used to store the discretevalues of the signal and associated transform parameters. Anotherchallenge is that signal recognition and signal alignment typicallyinvolves transforms and inverse transforms, which in addition tointroducing errors, are computationally expensive to implement inhardware, require additional memory, and introduce memory bandwidthconstraints as the need for read/write operations to memory increases aseach value in the discrete signal is transformed, re-sampled, orapproximated from neighboring sample values.

In view of these challenges, there is a need for methods to determinetransforms between signals that are accurate, yet efficient to implementin digital computing environments. This includes more effective ways toestimate linear transforms as well as determining translation or phaseshift.

One aspect of the invention is a method of determining a geometrictransformation of an image. The method operates on an electronic imagesampled from a physical object, with one or more processors programmedto execute the following method. The method starts with a seed set ofcandidate geometric transform parameters, which is distributed over aspace of candidate geometric transform parameters. For each of thecandidate geometric transform parameters, the method transformscoordinates of a digital watermark signal with the candidate geometrictransform parameters to produce a transformed watermark signal attransformed watermark signal coordinates. The method then determines ameasure of correlation between the transformed watermark signal and theelectronic image, for each of the candidate geometric transformparameters. It then selects a subset of the candidate geometrictransform parameters based on the measure of correlation.

This subset of candidates is then processed further to derive thegeometric transformation of the image. For a selected candidategeometric transform parameter, the method determines peak locationswithin the electronic image that are within neighborhoods around thetransformed watermark signal coordinates; updates locations of thetransformed watermark signal coordinates to the peak locations;determines a geometric transform that maps the digital watermark signalto the updated locations; and determines correlation between theelectronic image signal and the digital watermark signal, transformed bythe geometric transform. If the processing improves correlation betweenthe image and the transformed digital watermark, the method may continueto refine the geometric transformation.

The method refines the geometric transform by determining peak locationswithin the electronic image that are within a neighborhood ofcorresponding transformed watermark signal coordinates, updateslocations of the transformed watermark signal coordinates to the peaklocations, determines a new geometric transform that maps the digitalwatermark signal to the updated locations, and determines whethercorrelation has increased between the electronic image signal and thedigital watermark signal, transformed by the new geometric transform.The method may iterate this refinement processing so long as it yields ageometric transform that improves correlation.

Variants are implemented within a digital watermark detector asdescribed and claimed.

One technology described in this disclosure is a method of computing atransformation between a discrete reference signal with a suspect signalusing a direct least squares technique. The method provides a set offeature locations representing the discrete reference signal, andprovides a seed set of initial transform parameters. The featurelocations and transform parameters are represented as digital,electronic signals in an electronic memory. Using the seed set, themethod computes a least squares minimization that finds linear transformcandidates that minimize error when the linear transforms are used toalign the feature locations of the discrete reference signal andcorresponding feature locations in the suspect signal. This includescomputing a measure of correlation corresponding to the linear transformcandidates. The method evaluates the linear transform candidates foreach of the seeds to identify a subset of the candidates representingrefined estimates of linear transform candidates.

This disclosure also describes a direct least squares circuit. Thecircuit comprises a memory for storing a suspect signal representation.The circuit includes a correlation module for receiving a seed set oflinear transform candidates and determining a correlation metric foreach candidate as a measure of correlation between a reference signaland the suspect signal representation when the linear transformcandidate is applied.

The circuit also includes a coordinate update module for determiningfeature locations within the suspect signal representation of a featurethat corresponds to a feature of the reference signal at a locationdetermined by applying the linear candidate transform. This moduledetermines locations of components of a reference signal in the suspectsignal and provides input to a least squares calculator to determine thetransform between a reference signal and the suspect signal.

The circuit includes a least squares calculator for determining anupdated linear transform for each of the candidates that provides aleast squares fit between reference signal feature locations and thecorresponding feature locations in the suspect signal determined by thecoordinate update module. The circuit is implemented to use correlationmetrics to identify the most promising linear transform candidates. Forexample, the circuit iterates through the process of updating thetransform so long as the correlation metric shows signs of improvementin the transform's ability to align the reference and suspect signals.

Some embodiments employ a method of computing an estimate of phase of atransformed signal. This phase estimation method provides a set offeature locations representing a discrete reference signal, receives asuspect signal, and applies a transform to the reference signal toprovide a set of transformed locations. It samples phase from thesuspect signal at discrete sample locations in a neighborhood around thetransformed locations. To these sampled phases, the method applies apoint spread function to provide an estimate of phase of the suspectsignal at locations corresponding to the transformed locations.

Phase estimation is implemented, for example, in a digital logic circuitcomprising a memory for storing phase of a suspect signal and atransform module for transforming coordinates of a reference signal intotransformed coordinate locations. The circuit also comprises a pointspread function module for reading selected phase of the suspect signalfrom the memory at locations around a transformed coordinate locationand applying a point spread function to the selected phase to provide anestimate phase.

Some embodiments further employ a method of computing an estimate of atranslation offset between a reference and suspect signal. This methodoperates on a set of phase estimates of a suspect signal. For eachelement in an array of translation offsets, the method provides a set ofexpected phases of the reference signal at the translation offset. Itcomputes a phase deviation metric for each of the set of expected andcorresponding phase estimates at the translation offset, and computes asum of the phase deviation metrics at the translation offset. Thisapproach provides a phase deviation surface corresponding to the arrayof translation offsets. The method determines a peak in the phasedeviation metrics for the array of translation offsets (e.g., in thephase deviation surface), wherein a location of the peak provides theestimate of the translation offset.

This phase deviation method is implemented, for example, in a phasedeviation circuit. The phase deviation circuit comprises a memory forstoring a set of phase estimates of a suspect signal and known phases ofa reference signal. It also comprises a phase deviation module forcomputing a phase deviation metric for each of the set of known phasesof the reference signal and corresponding phase estimates from thereference signal for an array of translation offsets, and for computinga sum of the phase deviation metrics at the translation offsets. Thecircuit comprises a peak determination module for determining a peak inthe phase deviation metrics for the array of translation offsets. Thelocation of the peak provides the estimate of the translation offsetbetween the reference and suspect signals.

The above-summarized methods are implemented in whole or in part asinstructions (e.g., software or firmware for execution on one or moreprogrammable processors), circuits, or a combination of circuits andinstructions executed on programmable processors.

Further features will become apparent with reference to the followingdetailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an implementation of a processfor determining a transformation between a reference and suspect signal.

FIG. 2 is a diagram illustrating a hardware device that computes anaffine transform between a reference and suspect signal.

FIG. 3 is a flow diagram illustrating a least squares method thatcomputes a best fit transform that aligns a reference signal with itscounterpart in a suspect signal.

FIG. 4 is a block diagram of processing flow in a digital logic circuitimplementation.

FIG. 5 is a block diagram of a illustrating a method to computeestimates of the phases of a suspect signal for which an approximationof a linear transform has been computed.

FIG. 6 is a block diagram showing an alternative method to computeestimates of the phases of the suspect signal.

FIG. 7 is a diagram illustrating that the transformed frequencycomponent of a discrete reference signal does not fall on integercoordinates in a Fourier domain, and as such, requires a phaseestimation method to estimate phases from neighboring frequencylocations.

FIG. 8 is a diagram illustrating a process of deriving the phaseestimation method and associated mathematics supporting the derivation.

FIG. 9 is a flow diagram illustrating a phase estimation method tocompute phases given a linear transform and a reference signal.

FIG. 10 is block diagram illustrating a digital logic circuitimplementation of phase estimation.

FIG. 11 is a flow diagram illustrating an overview of a phase deviationmethod.

FIG. 12 is a block diagram of a digital logic circuit for estimating alinear transform.

FIG. 13 is a block diagram of a digital logic circuit for phaseestimation and phase deviation.

FIG. 14 is a diagram illustrating a phase deviation equation based on adeviation metric.

FIG. 15 is a diagram illustrating an implementation of a direct leastsquares method.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an implementation of a processfor determining a transformation between a reference and suspect signal.We refer to this process as a transformation recovery process because itrecovers a transformation of the reference signal from a suspect signalcaptured within a device. In particular, we have implemented this methodto recover the transform required to align the suspect signal with thereference signal. The process takes as input a discrete representationof a known reference and the captured suspect signal 100 and determinesan estimate of a transformation, which when applied to the referencesignal, would approximate the suspect signal. The transformationrecovery process is sub-divided into stages 102-106 in which the firststage provides an estimate of a transform (e.g., a linear transformdescribed by 4-D vector of linear transform parameters (or 2 by 2 lineartransform matrix)) and the subsequent stages refine the estimate byfirst estimating a phase of the reference signal as transformed by anestimated transform of stage 102 and then finding the phase and thetranslation in stage 106 (thus, providing two additional parameters oftranslation). In our particular implementation, stage 102 providescandidate linear transforms, each corresponding to a 2 by 2 lineartransform matrix. The next two stages provide two dimensional (2D)translation (in vertical and horizontal directions), which when combinedwith the 2 by 2 linear transform matrix, provide affine transformcandidates. In this context, the phase shift and translation are relatedquantities that are expressed in different domains—the phase shift as achange in phase angle of signal components in a Frequency domain such asa Fourier transform domain, and translation in the spatial (e.g., thespatial domain of an image) or temporal domain (time domain of timevarying signals like audio). Each of the stages 102-106 includes novelelements independent of the other stages, and we explain these novelelements in more detail. These stages are implemented in a system tocompute an affine transform between signals and provide additionaladvantages when used in combination as explained further below. Themethods and associated hardware implementations have application in avariety of signal detection and object recognition and matchingapplications. We illustrate examples of the methods in the context ofdigital watermark detection, but the methods are not limited to thiscontext.

The methods also apply to a variety of signal types. They are designedto operate on two dimensional content signals captured from sensors(e.g., images or video frames captured with cameras). The methods alsoapply to one dimensional as well as 2 or more dimensional signals. Oneimplementation, in particular, is adapted to determine the geometrictransformation of a reference signal in image content. The referencesignal is in the form of a two dimensional image watermark signal thatis embedded in a host image. The reference signal can be generalized toencompass a variety of signal types for different applications. As such,the software and hardware implementations have applications in a varietyof signal processing applications, such as object recognition, patternrecognition and matching, content fingerprinting, machine vision, andother applications where transformations between signals are computed.Our methods are particularly adapted for processing of signals capturedin sensors, and in particular, image sensors like CCD and CMOS arrays,of the type used in digital scanners, cameras, mobile telephonehandsets, etc.

As an example to provide context for the methods, we begin with anoverview of watermark signal detection hardware. We then describeimplementations of the individual stages. FIG. 2 is a diagramillustrating a hardware device that computes an affine transform betweena reference and suspect signal. This particular design is adapted torecover the affine transform of an embedded two dimensional watermarksignal. The design buffers portions of a captured and digitizedelectronic image in memory (RAM) 120 (the suspect image signal). Afilter and Fourier transform processing module 122 filters the suspectimage and computes a 2D Fourier transform. A linear transform estimationmodule 124 takes the discrete frequency representation of the suspectimage and computes an estimate of a linear transform between a referencesignal and the filtered suspect signal. Affine transform recovery module126 uses the linear transform estimate, and reference and suspect signalrepresentations to compute the phase/translation between the referenceand suspect signal and so results in an affine transform whichtransforms the reference signal to the suspect signal.

The lower portion of FIG. 2 provides a break-down of sub-modules withinmodules 122-126. Implementations of these sub-modules are describedfurther below.

The transform estimation of FIG. 1 can be implemented in a number ofalternative ways. One approach is to perform a matched filtercorrelation between the reference and suspect signals. One such methodfor determining rotation and scale of a reference signal relative to asuspect signal is a Fourier-Mellin correlation. By converting both thesuspect and reference signals to Fourier-Mellin coordinate space (a logpolar coordinate space), the rotation and scale transform between thetwo signals is converted to translation shifts, enabling the applicationof matched filter correlation to find the location of a correlationpeak, which corresponds to an estimate of the rotation and scale betweenthe signals. Another is to perform a least squares method, and inparticular, a direct least squares method. Below we describeimplementations of least square methods. These are particularly usefulfor implementation in hardware, where the processing can be implementedin sequential pipelined hardware logic stages, and in software where theprocessing can be performed in parallel on special purpose hardwareprocessing units such as, Graphics Processing Units (GPUs), DigitalSignal Processors (DSPs) or multi core Central Processing Units (CPUs),to name a few.

Least Squares

Least Squares Technique

The least squares technique estimates a linear transform that yields theleast square error (i.e., the maximum likelihood estimate), given aninitial guess of the linear transform. Operations consist of multipliesand adds, and are hardware friendly.

FIG. 3 is a flow diagram illustrating a least squares method. Oneimplementation takes as input the coordinates for a set of referencelocations (in either the frequency or spatial domain) and thecorresponding set of coordinates for a set of transformed locations(again, either in the frequency or spatial domain). For the sake ofillustration, we describe the technique for an implementation in whichthe reference locations correspond to features in the frequency domain,and in particular, peaks in the frequency domain.

This least squares method is performed iteratively and includes threesteps for each iteration. These three steps are illustrated in FIG. 3 inprocessing blocks 130, 132, and 134:

Computation of transformed frequency coordinates 130—In this stage, thetransformed frequency coordinates are computed using the initialtransform and the original (i.e., non-transformed) frequency coordinatesof the signal.

Coordinate update 132—in this step, more suitable locations for eachtransformed frequency is sought by searching the frequency magnitudeplane for the peak value around a small neighborhood of the transformedfrequency. At the end of this step, the coordinate of each transformedfrequency is updated if a more suitable peak location is found for thisfrequency. The optimal frequency coordinates computed in this processresult in locations that can no longer be simultaneously determined by asingle linear transform.

Transform update 134—in this step, an updated linear transform iscalculated from the updated coordinates using the least squaresformulation. This updated transform is used as an initial guess for thenext iteration. The least squares technique provides the transform thatminimizes the squared error between the original and transformedcoordinates. In particular, it provides the transform that minimizes, inthe sense of least squared error, the sum of the location errors. Thecomputation of the new transform from the errors is implemented asfollows:

A measure of correlation, called correlation strength, is computed foreach iteration. The correlation strength metric can be used to determineearly termination of iterations or to provide regularization.

In theory, the least squares technique can find the actual lineartransform between a reference and suspect signal starting from anyinitial guess of the linear transform parameters. However, from apractical standpoint (to prevent the coordinate update from being overlycomplex), the initial guess of the linear transform parameters must besomewhat close to the actual linear transform. Consequently, thetechnique is sensitive to the initial guess.

The initial guess of the transform can be as simple as a rotation andscale pair.

This least squares method can determine any arbitrary linear transform(i.e., including rotation, scale, shear, and, differential scale).

Direct Least Squares (DLS)

DLS is an efficient application of the least squares technique todetermine the linear transform between a suspect and a reference signal.Our particular implementation applies to images, and in particular, thesuspect image is a watermarked image, and the reference signal is awatermark signal, which is assumed to be embedded in the watermarkedimage. The task, in this case, is to determine the linear transformbetween the original reference signal, which is known, and itscounterpart which is assumed to be embedded in the suspect signal.

In DLS, the least squares technique is applied to a sparse set ofinitial guesses of the linear transform.

DLS requires fewer evaluations than a Fourier-Mellin type correlation,while providing a more accurate transform than Fourier-Mellin. As notedabove, a correlation between reference and suspect signals in theFourier-Mellin domain provides an estimate of rotation and scale. Leastsquares, in contrast, can provide any arbitrary linear transform (e.g.,a 4D vector of linear transform parameters).

With DLS, the 4-dimensional space covered by the 2×2 linear transformmatrix can be evaluated extremely efficiently with initial guessesspanning a sparse 2-dimensional subspace.

Each DLS evaluation uses the least squares technique, and is independentof other DLS evaluations on the 2D subspace. Therefore, DLS evaluationscan be performed efficiently in hardware or on multi-core processorarchitectures. Each evaluation results in an estimated linear transformand a corresponding correlation strength value. Candidate lineartransforms are identified as those transforms corresponding to thelargest correlation strength values. One or more of these candidatelinear transforms are processed further to recover the affine transform.

DLS allows the initial guesses to be arbitrarily spread around the 2Dsubspace. For example, if the initial guesses comprise rotation/scalepairs, the spacing along the rotation axis and the scale axis can bearbitrary. In comparison, the Fourier-Mellin approach requires thespacing in the scale axis to be logarithmic. The use of arbitraryspacing has two advantages—increased robustness and efficientcomputation. In the general case, the set of initial guesses are ideallyselected such that they are uniformly distributed over a sparse subsetof rotation and scale values. For example, a uniform spacing in thescale axis (uniform increments in scale) can be efficiently computed andalso reduces noise artifacts. The ability of DLS to converge on theappropriate linear transform and the accuracy of the estimated transformis influenced by the number of initial guesses and the number of leastsquares iterations. Optimal values for these parameters are determinedas a tradeoff between hardware cost, computational speed and desiredrobustness. A more sophisticated strategy consists of using a verysparse set of initial guesses in conjunction with an adaptive number ofiterations. More iterations are performed for cases where the resultingtransforms from successive iterations exhibit convergence. This strategyprovides computational efficiency without sacrificing robustness.

In certain applications, the suspect signal may undergo a limited set oftransformations. For example, the rotation may be restricted to a rangebetween −30 and +30 degrees. In such situations, DLS evaluations areperformed on a further restricted range of sparse initial guesses.

Due to noise and distortion, the linear transform estimated by DLS canbe noisy. In our particular case, the transform is noisy when estimatedfrom a single image block of an image with weak watermark signal. Toreduce the noise in the estimated transform, we take advantage ofcharacteristics of the DLS output. Recall that DLS results in anestimated linear transform and a correlation strength value for eachinitial guess. For a well-designed set of initial guesses, multipleinitial guesses lead to similar linear transforms. In other words, theoutput linear transforms are clustered. To reduce noise in the lineartransform estimate, clusters of linear transforms are identified, andtheir elements appropriately averaged. Appropriate averaging can be doneby weighting each linear transform by some function (e.g. nonlinearfunction) of the correlation strength.

FIG. 4 is a block diagram of processing flow in a digital logic circuitimplementation. A Fourier transform module 140 accesses the input signal(e.g., block of image data captured by an image sensor) from memory, andcomputes a Fourier transform and Fourier magnitude data. Fouriermagnitude filter 142 filters the 2D Fourier magnitude data. One suchfilter is a non-linear filter that compares a sample value with each ofits eight neighbors and replaces the sample value with an aggregatevalue based on these comparisons. In one implementation, the filter usesa ratio of the sample value to the average of its 8 neighbors (in theFourier magnitude domain). The output of the filter is then a function(in this case, a nonlinear function) of this ratio. This filter isparticularly useful in extracting reference signal features (e.g., indigital watermarking where the reference signal is embedded in thesuspect signal). The output of the filter then forms the input to thedirect least squares method.

Update Coordinates (block 132 of FIG. 3)

The coordinate update process comprises a local search for a peak (ordesired characteristic such as a corner or feature) around a smallneighborhood surrounding the transformed location (frequency) ofinterest.

Neighborhoods are typically defined as a 3×3 or 2×2 region of samples orcould be much larger depending on the problem domain and application.

If the peak or desired feature is in a different location than thetransformed location, the coordinate of the transformed location isupdated to this location

The least squares method provides an estimate of the linear transformbetween the suspect and reference signals. To recover the completeaffine transform, the phase shift (or the translation) between the twosignals needs to be computed. One approach is to compute a phasecorrelation between a phase representation of both the reference andsuspect signals, taking into account the linear transform. We havedeveloped processing modules that are particularly advantageous inrecovering the affine transform. These processing modules, as notedabove in FIG. 1, are phase estimation and phase deviation.

Phase Estimation

Our phase estimation approach is advantageous because it calculatesphases from the Fast Fourier Transform (FFT) of a transformed signalrather than performing the inverse transform of the image followed by anadditional FFT to compute and extract the phases. Phase estimation usesthe linear transform that the reference signal has undergone within thesuspect signal. While direct least squares is illustrated as one methodto compute this linear transform, there are other ways to compute it,such as using matched filters (e.g., the Fourier Mellin correlation toapproximate the linear transform).

Highlighting this advantage, FIGS. 5 and 6 are block diagramsillustrating different methods of calculating phase information: onewithout phase estimation and the other with phase estimation. Bothmethods use direct least squares methods to estimate linear transformsbetween the signals. A first FFT is performed to obtain the magnitudeinformation that the direct least squares technique operates on. Theapproach of FIG. 6 uses our phase estimation technique, while FIG. 5performs an inverse linear transform followed by a second FFT tocalculate the phases. Phase estimation avoids the extra processing ofboth the inverse linear transform and the second FFT.

FIG. 7 is a diagram illustrating the problem that phase estimationaddresses. When the linear transform is applied to a reference signalcoordinate, it likely does not map to a discrete coordinate. The phaseestimation method provides an efficient approach to computing the phasesat non-discrete (i.e., real valued) co-ordinate locations.

To understand how phase estimation addresses the problem, we begin witha derivation of the phase estimation method. FIG. 8 is a diagramillustrating a process of deriving the phase estimation method andassociated mathematics supporting the derivation. As illustrated in FIG.7 and block 180 in FIG. 8, the first step in the derivation assumes afunction consisting of complex frequency situated at real position v inthe Fourier plane.

Block 182 of FIG. 8 and the corresponding mathematical expressionsillustrate a derivation of a Point Spread Function (PSF) used for phaseestimation. The PSF is derived by decomposing complex frequenciessituated at real positions in the Fourier plane in terms of integercomplex frequencies. This PSF is complex-valued.

As shown in block 184, the last step in the derivation performs aconvolution with PSF in Fourier Plane. The inner product of block 182 ofFIG. 8 is with respect to the Fourier basis functions—this provides thePSF. The PSF is then used in block 184 to convolve with the values inthe Fourier plane.

FIG. 9 is a flow diagram illustrating a phase estimation method tocompute phases given a linear transform and a reference signal. Thereare two principal stages in our phase estimation process. In a firststage, the implementation transforms the known reference signalcoordinates (in particular, a set of sinusoids at known frequencycoordinates) according to a linear transform. In a second stage, theimplementation uses the transformed coordinates and the phaseinformation surrounding these coordinates in the suspect signal'sfrequency domain to obtain phase estimates of the transformed frequencycoordinates. The inputs to this process are the discrete suspect signal,which is stored in memory in the form of a complex frequency plane fromthe 2D FFT, along with the assumed transform (e.g., the linear transformcomputed previously), and the reference signal's frequency specification(this is the set of known frequency locations of the sinusoids of thereference signal). For each real complex frequency, phase estimationapplies the following steps

a. Compute transformed real location (e.g., non-integral) of thefrequency using the provided linear transform (block 190).

b. Express complex frequency at the real location in terms ofinteger-coordinate Fourier basis. This provides the complex PSF (block192).

c. Obtain the phases for the integral frequencies surrounding thedesired real frequency from the FFT of the suspect image block (block194). The PSF is peaked at Delta=0, and so a non-integral peak shows upin a small neighborhood (as expected). In particular, the function(sin(pi Delta)/N sin(pi Delta/N)) has a peak at 0 (in the limit).

d. Compute the sum of products of the complex values at the integralfrequencies with the corresponding complex values of the PSF (block196). This gives the estimated phase at the real complex frequency.

The PSF values can be pre-computed and stored in a table for efficiency.In addition, the phases can also be quantized (to a few phase angles)for efficiency. The implementation of the first stage of phaseestimation makes a transformation to move each frequency location of thereference signal to the appropriate “fractional” position between thediscrete frequency samples. The characteristics of the transformedreference signal's phase are independent of the signal frequency. Foreach fractional frequency position, the PSF table contains pre-computedphase information for the nearest discrete frequency locations.

To simplify the computation, the implementation uses a limitedresolution of the fractional frequency positions, between each integerfrequency. The implementation uses this reduction in number offractional frequency positions to further reduce the size of the PSFtable. The PSF table contains pre-computed phase information only foreach permitted fractional frequency position.

This PSF phase information is then re-used for all future estimations(in the 2nd stage of the process). In one particular implementation, thephase information is pre-computed and the values are stored in smalldiscrete tables. The tables are the same for horizontal and verticalfrequency directions, so the implementation accesses twice and combinesthe values to make the expected phase for a 2D frequency location.

Our phase estimation operations are efficient and hardware friendly.Besides eliminating the inverse transform and additional FFT, thisapproach does not require access to the suspect signal data (e.g., theinput suspect image) as shown in the method of FIG. 5. Instead, it usesthe frequency data of the suspect signal, which has already beencomputed, as shown in FIG. 6. Consequently, Phase estimation lendsitself to a pipelined architecture in hardware.

In general, the phase estimation technique can be used to performrotations or other transformations in the complex frequency domain,without first resorting to the spatial domain data.

FIG. 10 is a block diagram illustrating a digital logic circuitimplementation of phase estimation. The phase estimation implementationshown in FIG. 10 receives a stream of phase information for a suspectsignal block at data in module 200. Under control of control module 202,it stores the phase information (the phase half plane from a 2D FFT ofthe suspect signal block) in RAM memory 204. Linear transform candidatesare also received through data in module 200 and stored directly inmodules 206, 208 and 212 (alternatively, could be stored in RAM 204).

Linear transform candidate matrix module 206 forms the linear transformcandidates in a matrix and provides them to a matrix invert module 208and data out module 210. Matrix invert module 208 inverts the lineartransform matrix. In this implementation, the linear transform is for aspatial transform of the image. For mapping the reference signalcoordinates in the suspect image frequency domain, it takes the inversetranspose of the linear transform. Transform coordinate module 212 thentakes a reference signal coordinate specifying the location of areference signal component from a memory (Read Only Memory (ROM) 214)and transforms the location to a coordinate in the coordinate space ofthe suspect signal block. Control module 216 sequences through each ofthe locations of the reference signal components, providing thecoordinates in the frequency domain. For each reference signalcoordinate, control module 218 sequences through a 2 by 2 matrix ofPoint Spread Function (PSF) points. As it does so, it controls acoordinate ceiling/floor function module 220 that operates on thetransformed coordinate of the reference signal component, and it selectsthe PSF for that coordinate in PSF table 222. The coordinateceiling/floor module 220 then selects the neighboring frequencylocations in the phase information RAM 204, which in turn, outputs thephase information at the neighboring locations to product and sum logicoperators 224. The product and sum operators 224 apply the point spreadfunction from table 222 to the phase information to calculate theestimated phase. Data out module 210 then outputs the estimated phasesfor each reference coordinate of the reference signal, along with thecorresponding linear transform candidate. The phase estimationimplementation cycles through all of the linear transform candidates,providing a set of estimated phases for each reference signal componentfor each LT candidate.

Phase Deviation

Referring back to FIG. 1, the process following phase estimation is touse this estimate of the phase of the transformed reference signal todetermine the translation between the reference and suspect signals.There are alternative approaches to computing the translation at thispoint. One approach is to perform phase correlation between the phaserepresentations of the transformed reference signal and the suspectsignal (this requires an inverse FFT operation). Below, we describe analternative approach referred to as phase deviation.

2D Phase Deviation

Phase deviation is an alternative approach to estimating the translationbetween two images or signals in general. As compared to a phasecorrelation approach, it does not require the inverse FFT operation.

FIG. 11 is a flow diagram illustrating an overview of a phase deviationmethod. The phase deviation method first obtains a set of candidatetranslation values (called the translation offsets) at a first level ofdetail (e.g., integer offsets) between the reference and suspect signalsin step 230. In step 232 of FIG. 11, the phase deviation method refinesthese candidates by determining translation values that provide a betterfit between the reference and suspect signals using a higher level ofdetail (e.g., fractional offsets) around the first set of candidatetranslations.

The phase deviation for a specified translation offset is the sum ofdeviations between the measured and the expected phases at all referencesignal components of interest. In the case where the reference signalcomprises a set of sinusoids, each with particular phase, the expectedreference signal phases are the phases of the sinusoids at knowntranslation offsets. These expected phases are provided for each ofseveral translation offsets, which may be specified in terms of a phaseangle or translation value (e.g., pixel offset at a particular imageresolution). Stated another way, for each possible translation offset,there is a set of expected phases for the reference signal.Additionally, the other input is the measured phases, previouslycomputed from the suspect signal. The deviation between the expectedphases and the measured phases is computed for each translation offset.The deviation at each frequency can be calculated using a distancemeasure such as Euclidean distance between the measured and expectedphases. The phase deviations calculated for all possible translationoffsets constitute the 2D phase deviation surface. The location of theminimum value in the 2D phase deviation surface indicates the locationof the translation offset.

A 2D phase deviation method can be implemented using just adds (nomultiplies), and at a fraction of the computational cost of a 2D FFT.Also, the phase deviation calculations for each offset and for eachfrequency can be computed independently, leading to efficient parallelimplementations. This is an advantage over alternative methods, likephase correlation.

The phase differences and deviations can either be computed as complexvalues or can be computed directly in terms of angles. Working withangles provides improved efficiencies in computation.

Distance measures other than the Euclidean distance can also be used.For example, the L1 norm or a nonlinear measure can provide improvementsdepending upon the specifics of the signals and noise involved.

In particular, the sum of deviations may be computed as the sum ofabsolute differences between the measured and expected phase angles,where each difference is wrapped between −pi and +pi, (modulo 2*pi).This computation is efficient to implement in hardware.

Sub-Sample Translation Estimation

Phase deviations can be computed for any arbitrary real valuedtranslation offsets. This provides sub-sample translation estimation asopposed to integer valued translation estimation with the phasecorrelation approach.

The ability to compute a phase deviation metric at sub-sampletranslations can be used to implement a translation refinement techniquewhere integer translations are first evaluated to determine suitabletranslation offsets around which further refinement is performed byevaluating sub-sample (i.e. fractional, sub-pixel for image content)translation offsets.

One Dimensional (1D) Phase Deviation

The basic phase deviation formulation can be modified to exploitpatterns in the frequencies. Sets of frequencies for which the linearcombination of coordinate values in one dimension (horizontal orvertical) is zero, lead to a 1D phase deviation formulation in theorthogonal dimension. Conceptually, this leads to hypothetical 1Dsignals in the orthogonal dimension which are a multiplication of thesets of 2D sinusoids in 2D space. The frequency of the hypothetical 1Dsignal is given by the sum of frequencies in the orthogonal dimension.Translation can then be estimated independently in each dimension usingthe 1D phase deviation formulation, for a fraction of the cost of 2DPhase Deviation. Besides, the search for a minimum phase deviationmetric is along 1D (i.e. is on a one dimensional data set), furtherreducing the overall computational cost.

In some cases, the linear combinations lead to hypothetical 1D signalsthat are outside the support length (e.g. 128 points) of the originalsignal. These hypothetical 1D signals have a higher frequency thanNyquist. In this case, a 1D phase deviation method can be specified interms of a larger artificial support length (e.g., using 256 points toensure a higher sampling rate) to avoid aliasing. Avoiding aliasingincreases reliability of translation estimation in noise.

1D phase deviation causes ambiguities in translation when all theresulting hypothetical frequencies in the orthogonal direction are evenvalued. For example, when pairs of quadrant symmetric frequencies oflength 128×128 in 2D space (such as [−45, 9] and [45, 9], and, [−44, 6]and [44, 6]) are multiplied, the resulting 1D phase deviation has aperiodicity of length 64. The frequency doubling caused by combining twofrequencies of the same value leads to even valued 1D signal frequencies(e.g., 18 and 12), thereby introducing ambiguity. As a corollary toaliasing, mixing two frequencies A and B, produce new frequencies A+Band A−B. The ambiguity caused by periodicity can be resolved using 2Dphase deviation for further evaluation of specific translations.Alternatively, the ambiguity can be avoided by ensuring that asubstantial number (around half) of the hypothetical frequencies are oddvalued.

A combination of 1D phase deviation and 2D phase deviation can beemployed to take advantage of the meager computational load of 1D phasedeviation and the robustness of 2D phase deviation.

FIGS. 12 and 13 are block diagrams illustrating hardware implementationsin more detail. FIG. 12 is a block diagram of a digital logic circuitfor estimating a linear transform (e.g., block 102 in FIG. 1 and blocks122-124 in FIG. 2). FIG. 13 is a block diagram of a digital logiccircuit for phase estimation and phase deviation (e.g., blocks 102-104in FIG. 1 and block 126 in FIG. 2).

As shown in FIG. 12, the input to the implementation is a packet ofimage data from the suspect image. The implementation computes theaffine transform of a digital watermark signal embedded in the inputimage, which is the suspect image, relative to the initial coordinatespace of the digital watermark, which is the reference signal. In thisparticular example, the reference signal is a set of frequency peakscorresponding to the watermark signal (namely, a set of sinusoids with aknown, pseudorandom phase relative to each other). At this point in theprocess, the suspect image may have been subjected to various forms ofdistortion caused by sampling (scanning, printing, etc.) as well asgeometric distortions (e.g., as a result of image editing and/or captureof the image in a transformed state from a scanner or camera). As aresult of this distortion, the affine transform that best approximatesthe transformation between the known reference signal and itscounterpart embedded in the suspect image is not known. The objective isto compute the affine transform that best approximates thetransformation between the reference signal at the time of embedding,and the embedded reference signal within the suspect image.

Before describing the circuit implementation, it is helpful to providebackground on the attributes of the reference and suspect signalsbecause they dictate design considerations for the hardware. The digitalwatermark has been repeated within adjacent blocks (e.g., in a tiledfashion) of the signal. The digital hardware circuitry operates on astream of input packets. The input packets comprise overlapping blocksof the suspect image that roughly correspond to the original size of theblocks into which the watermark was originally embedded. Each block is a128 by 128 array of pixels. The size of memory and FFT filters etc. areadapted based on these signal attributes, and can vary with theapplication and signal specifications for those applications.

The pre-filter 300 filters the pixel values within the image block usingthe method described previously. Namely, each sample is compared withits eight neighbors and replaced by a value that is a function of thesecomparisons to provide a form of non-linear filtering that seeks toisolate the embedded reference signal from the suspect image data.

The window operation 302 prepares the filtered image data for a 2D FFT.The resulting filtered image data block is received by FFT2D (304) andstored in RAM. In this case, the RAM (306) is implemented within an ASICalong with the other hardware components shown in FIG. 12. FFT2Dprocesses a block of spatial input data to produce complex frequencydata. The Real and Imaginary parts of complex frequency data areinterleaved in output into a single pipe output stream.

CORDIC 308 converts interleaved Real (Re) and Imaginary (Im) stream intointerleaved magnitude and phase stream. As known in the field, CORDIC isa method for efficient digital signal processing implementation oftrigonometric functions. A Fourier Magnitude Filter 310 filters only theFourier Magnitude portion of the data. The filter uses a ratio of thesample value to the average of its 8 neighbors (in the Fourier magnitudedomain). The output of the filter is then a function (in this case, anonlinear function) of this ratio. The phase is passed throughun-altered.

The Direct Least Squares (DLS) module 312 receives an interleaved streamof the Filtered Fourier Magnitude and Phase data. Each of these datastreams is stored in a RAM, shown as RAM blocks 314 and 316.

DLS computes and refines each potential linear transform candidate formaximum correlation strength. The output of the DLS module 312 is astream of linear transform (LT) candidates, preceded by the stored phaseblock. Phase data used for phase estimation is stored in a form that isready to be sampled so that phases can be estimated for each candidatelinear transform.

Block 318 sorts the input stream of linear transform candidates to findthe top 10 candidates, based on a measure of correlation. This measureof correlation, in this implementation, is a correlation strengthcomputed as the dot product between the reference and suspect signalsafter the linear transform candidate is used to align these signals. RAM320 is a memory used to store the top linear transform candidates andcorresponding correlation metrics.

FIG. 13 starts where FIG. 12 ends with the top linear transformcandidates. The phase estimation module 322 receives the stream of phasedata and stores it in RAM 324. It uses each of the linear transformcandidates to estimate a set of phases for signal components in thesuspect image corresponding to each of the frequency locations in thereference signal. For each linear transform candidate, the phaseestimation module provides both the linear transform candidate and a setof phases corresponding to the frequency locations in the referencesignal. These phases represent a measure of the phases of the referencesignal component that is embedded in the suspect signal. In particular,for this implementation where the reference signal is embedded into thesuspect signal as a digital watermark, the set of phases represent theestimates of the phases of the embedded reference signal components,which correspond to sinusoids with random phase.

In other implementations, the phase estimation module may be subsumedwithin the DLS module, since much of the matrix calculations totransform reference signal coordinates are already computed there, andthe phase data is also readily available. This will result in the DLSmodule outputting both linear transforms and estimated phases for eachof those transforms.

While the phase estimation method is depicted for a digital watermarkdetector implementation, the method is applicable to other applicationswhere a signal processor seeks to find a known reference signal within asuspect signal. Examples include object recognition and patternmatching, where the signal processor seeks to find a known referencesignal in an image. The phase estimation method enables the signalprocessor to compute estimates of the phase of a reference signal thatis suspected to be a component of the suspect image. These phaseestimates can then be used in additional matching or recognitionoperations to detect whether the reference signal is present in thesuspect signal. In these methods, the same general approach is followed:the phase estimation uses an estimate of the transform between anexpected signal pattern and corresponding components in a suspectsignal, along with the phase of the suspect signal to compute estimatesof the phase of the signal pattern in the suspect image.

Returning to FIG. 13, the phase deviation module 326 receives eachlinear transform candidate and a corresponding set of estimated phasesof the reference signal in the suspect signal. The phase deviationmodule 326 computes a phase deviation surface for each linear transformcandidate. This surface is an array of phase deviations, where eachelement in the array corresponds to a translation offset and the valueof the element is sum of phase deviation metrics between correspondingexpected and measured phase differences. For 2D phase deviation, this isa 2D array of phase deviation values corresponding to all pairs oftranslation offsets (e.g., a surface of 128 by 128 values). As describedpreviously, the phase deviation for a particular translation offset iscomputed as a sum of a difference metric that calculates the deviationbetween an expected phase difference and the measured phase differenceat a particular reference signal component. For our implementation,there are four orientations for each linear transform candidate,corresponding to orientations of 0, 90, 180 and 270 degrees. At the endof computing the phase deviation surface for an orientation, phaseregisters are re-oriented by 90 degrees.

The objective of the 2D phase deviation module is to provide one or moretranslation offset candidates corresponding to minima in phasedeviation. Stated another way, the objective is to find the translationoffset that best matches the expected and measured phase differences, asdetermined by the minimum deviation between the two. The implementationsubtracts the phase deviation from a large constant to convert theproblem of searching for minima to a problem of searching for peaks forconvenience (in this case, a peak represents a minimum phase deviationmetric in the phase deviation surface). Since the objective is to findthe best matches between the expected and measured signals (i.e. theknown reference signal and its counterpart in the suspect image), thehardware seeks to find peaks in the deviation between the two. Theinitial 2D phase deviation surface is computed for integer translationoffsets for computational efficiency. However, the actual translationmight lie at a fractional (i.e., sub-pixel) offset. As a result, peaksin the inverted phase deviation surface might be spread over a 2 by 1pixel region (in either the horizontal or vertical direction). Toovercome this effect, peaks are searched over 2×1 regions in theHighest2×1 module 328.

To overcome the effects of noise and distortion, the top N peakcandidates are further evaluated using a refinement module 332.

The refinement module begins with the top N peaks (e.g., 2 by 1) peaksidentified in the inverted 2D phase deviation surface (e.g., thegreatest minimum in magnitude in the phase deviation surface). The valueof N is determined as a tradeoff between computational efficiency androbustness and is typically between 2 and 10. Then, for each of these Npeaks, it computes a refined phase deviation surface in a neighborhoodaround the translation offset corresponding to the peak. These refinedphase deviations are computed for sub-pixel translation offsets. Inparticular, the expected phase differences are computed for each of thesub-pixel translations in a M by M array around the integer translationoffset location of a peak. The value of M and the fractional (i.e.,sub-pixel) increments in translation are determined based on the desiredcomputational throughput and robustness. A typical value for M is 16,while a typical fractional increment is a quarter pixel. The sum ofphase deviations is calculated to provide the sub-pixel phase deviationsurface. If there is a sub-pixel offset with a higher peak, thissub-pixel offset is included in a list of the top peaks.

The output of the 2D phase deviation module is a linear transformcandidate followed by a list of peak coordinates corresponding to minimain the phase deviation surface (including any surfaces computed in therefinement stage).

The translation correction module 330 corrects the translation offsetcomputed for each of the linear transform candidates. The nature of thecorrection is specific to the implementation and depends onimplementation details such as, whether the reference signal phases usedas inputs are expressed relative to Fourier representation block centeror block corner, and Fourier processing and representation relative toblock center or corner, as well as differences in the translationdepending whether it is represented relative to the coordinate system ofthe transformed suspect signal or the reference signal.

FIG. 14 is a diagram illustrating a phase deviation equation based on adeviation metric. This diagram provides an example of a phase deviationmetric. This metric is one example illustrating how to compute phasedeviation values in the phase deviation surface. As shown in FIG. 14,phase deviation represents the deviation between a measured phasedifference and expected phase difference for a particular frequencycomponent, i, of the reference signal. The measured phase difference isthe difference between the phase angle at frequency component, i, forthe estimated phase of the reference signal in the suspect signal (e.g.,as determined by the phase estimation process) and the known phase angleof the reference signal component. As noted previously, the phaseestimation process provides a phase angle estimate for the suspectsignal in the transformed state. In the implementation, the phase angleused for the known reference signal is in its original, un-transformedstate.

The expected phase difference is directly computed from the horizontaland vertical translation offsets. As noted, these offsets start out asinteger offsets, and then are sub-integer (e.g., sub-pixel) forrefinement.

Note that in the equation that there are M frequency components in thereference signal. The deviation metric is a sum of the individualdeviations for each of the frequency components. While the Euclidiandistance measure is shown, other deviation metrics may be used aspreviously indicated.

From the depiction in FIG. 14, one can see that the 2D case shown can bereduced to 2 separate instances of 1D phase deviation by using areference signal that has pairs of frequency components that aresymmetric about the vertical axis, and thus, the horizontal componentscancel each other, and likewise, have pairs of frequency components thatare symmetric about the horizontal axis, and thus, the verticalcomponents cancel each other. As noted, this enables the vertical andhorizontal translation offsets to be determined separately inindependent searches for the peak in the respective 1D phase deviationarrays.

FIG. 15 is a diagram illustrating an implementation of a DLS method.This is an implementation of the processing within, for example, block312, in FIG. 12. As noted previously, the DLS module begins with a setof seed linear transform candidates in block 350. For example, thisimplementation begins with a sparse set of rotation-scale candidates (asnoted in one example above), which comprise subset of the lineartransform parameters represented in a 2 by 2 linear transform matrix.The other linear transform parameters represented in a 2 by 2 matrixinclude differential scale (e.g., horizontal and vertical scales) andshear (e.g., horizontal and vertical shear). When the DLS method startswith rotation and scale candidates, the other parameters are initiallyassumed to not provide any additional transformation, and subsequentiterations of the method update the linear transform in a manner thatenables the other linear transform parameters to vary so as to provide abetter fit between the reference and suspect signal. In otherimplementations, a different subset or formulation of sparse lineartransform candidates may be chosen as seed candidates.

For each linear transform candidate in the starting set of seedcandidates, a transform module in the DLS module transforms thefrequency locations of the frequency components in the reference signal(352). A sample module then samples the frequency plane of the suspectsignal at locations in a neighborhood around the location of eachtransformed location (354). The neighborhood is a region surrounding thetransformed frequency location, and in practice it encompasses somenumber of discrete frequency locations in the FFT of the suspect signal.Next, a correlation module in the DLS module computes a signalcorrelation metric (356) that provides a measure of correlation betweenthe reference signal and the suspect signal for these regions in thesuspect signal around each transformed component of the referencesignal. At the end of this process, the DLS module has a signalcorrelation metric for the linear transform candidate. It determineswhether this metric is improved relative to a previously stored metricfor a prior iteration (358). The DLS module continues so long as thereis improvement in the correlation metric (358) and an iteration limit isnot met (360).

There are a variety of ways to compute regions and the signalcorrelation metric computed for those regions. In one implementation inwhich the DLS module samples from the four nearest neighbor locations,the signal correlation metric is computed as a sum of the bi-linearinterpolation of the frequency magnitudes at those neighbors for eachtransformed location within the suspect signal. Alternatives includecomputing correlation using a bi-cubic interpolation, and using a 3 by 3sample region around each transformed frequency component's location.The correlation can also incorporate a correlation of the phasecomponents of the reference and suspect signals at the regions. In thiscase, the phases in the suspect signal are estimated using the phaseestimation method.

In the case where the DLS module finds a linear transform candidate thatimproves upon the signal correlation and is below the iteration limit,the DLS module proceeds to establish a set of inputs to a least squarescalculator, which in turn, computes a new candidate linear transform.This set of inputs comprises a set of frequency component locationscorresponding to each transformed frequency component location, wherethere is a maximum in frequency magnitude. The process of finding thesenew locations for each component of the reference signal is reflected inblock 362. In particular, a coordinate update module computes featurecoordinates (e.g., peaks) in a neighborhood around the transformedcoordinate locations. Next, the least squares calculator (364) computesa new linear transform candidate by using the least squares method tofind a linear transform that best maps the reference signal componentsfrom their original locations to the new locations found in block 362.

The process depicted in block 362 is an implementation of “CoordinateUpdate” discussed above. One approach to updating the coordinates of afrequency component of the reference signal is to select the coordinatesof the neighboring frequency location with the maximum magnitude in aneighboring region, such as a 2 by 2, 3 by 3, 5 by 5, etc. sample regionaround the transformed frequency location. This process does not requireinterpolation to find new coordinates. In some implementations, we havefound that a 3 by 3 neighborhood covers differential scale up to 2-3%and sometimes up to 5%. There is a trade-off between using a largerneighborhood and potential confusion due to noise of adjacent frequencycomponents of the reference signal. Our implementations use a referencesignal where M is in the range of 35-75, the suspect image is sampledaround a resolution of 100 dots per inch (DPI), and the block size andFFT size is 128 by 128 samples. The neighborhood sizes and shapes can betailored for the unique characteristics of the reference signal.Neighborhood sizes can increase with increasing frequency. Theneighborhood size and shape can be tailored to avoid conflict of noisedue to adjacent frequency components in the reference signal. Theneighborhood size and shape can also be adapted as a function of thelinear transform candidate (e.g., transformed by the LT candidate). Theupdate to the coordinate of a transformed location can also be computedas combination of neighboring values, such as by finding the center of aneighboring peak (e.g., a Normalized Center of Mass), a Center of Mass,a quadratic fit, or other interpolation of neighboring values.

The least squares calculator of block 364 implements the expressionshown in the diagram to solve for the 2 by 2 linear equation on the lefthand side of the expression. This is implemented in hardware usingmultiply and add logic circuitry, and of course, can be implemented insoftware (including firmware instructions). As shown, the inputs are thecoordinates of the reference signal components and the correspondingupdated coordinates for the reference signal in the suspect signal asdetermined from the previous block (362).

After computing the update of the linear transform candidate in block364, the DLS modules adds this linear transform as a new candidate andreturns to block 352.

When the DLS module completes as determined in decision blocks 358-360,the resulting linear transform candidate and its associated signalcorrelation metric are stored for further processing (366). The DLSmodule repeats for additional seed linear transform candidates as shownin block 368. When the initial candidates have been processed andrefined as shown, the DLS module has a refined linear transformcandidate for each initial seed candidate. It searches this set ofrefined linear transform candidates for the strongest correlation. Asubset of the top candidates based on correlation can then be used infurther processing as noted. Also, as noted previously, linear transformcandidates can be clustered and combined to form new linear transformcandidates.

Concluding Remarks

Having described and illustrated the principles of the technology withreference to specific implementations, it will be recognized that thetechnology can be implemented in many other, different, forms. Toprovide a comprehensive disclosure without unduly lengthening thespecification, applicants incorporate by reference the patents andpatent applications referenced above.

The methods, processes, and systems described above may be implementedin hardware, software or a combination of hardware and software. Forexample, the signal processing operations for DLS, phase estimation andphase deviation may be implemented as instructions stored in a memoryand executed in a programmable computer (including both software andfirmware instructions), implemented as digital logic circuitry in aspecial purpose digital circuit, or combination of instructions executedin one or more processors and digital logic circuit modules. The methodsand processes described above may be implemented in programs executedfrom a system's memory (a computer readable medium, such as anelectronic, optical or magnetic storage device). The methods,instructions and circuitry operate on electronic signals, or signals inother electromagnetic forms. These signals further represent physicalsignals like image signals captured in image sensors, audio captured inaudio sensors, as well as other physical signal types captured insensors for that type. These electromagnetic signal representations aretransformed to different states as detailed above to determine lineartransforms, phase shift and translation between signals.

The above methods, instructions, and hardware operate on reference andsuspect signal components. As signals can be represented as a sum ofsignal components formed by projecting the signal onto basis functions,the above methods generally apply to a variety of signal types. TheFourier transform, for example, represents a signal as a sum of thesignal's projections onto a set of basis functions.

The particular combinations of elements and features in theabove-detailed embodiments are exemplary only; the interchanging andsubstitution of these teachings with other teachings in this and theincorporated-by-reference patents/applications are also contemplated.

1-12. (canceled)
 13. A method of determining a geometric transformationof an image, the method comprising: receiving an electronic imagesampled from a physical object; with one or more processors: determininga candidate geometric transform between a digital watermark signal andthe electronic image; refining the candidate geometric transform bytransforming coordinates of the digital watermark signal with thegeometric transform, determining peak locations within the electronicimage that are within a neighborhood of the transformed coordinates ofthe digital watermark signal, updating locations of the transformedcoordinates to the peak locations, determining a new geometric transformthat maps the digital watermark signal to the updated locations, anddetermining whether correlation has increased between the electronicimage and the digital watermark signal, transformed by the new geometrictransform; from the electronic image, estimating phase at non-integercoordinates corresponding to coordinates of the digital watermark signaltransformed with the new geometric transform; and determining deviationbetween estimated phase and expected phase of the digital watermarksignal.
 14. The method of claim 13 wherein the candidate geometrictransform comprises two or more affine transform parameters.
 15. Themethod of claim 14 wherein the two or more affine transform parameterscomprise rotation, differential scale and shear.
 16. The method of claim13 wherein the digital watermark signal comprises peaks at thecoordinates of the digital watermark signal.
 17. A digital watermarkdetector comprising: a memory for storing an electronic image sampledfrom a physical object; a processor in communication with the memory,the processor configured to obtain a candidate geometric transform, andconfigured to refine the candidate geometric transform by transformingcoordinates of the digital watermark signal with the geometrictransform, determining peak locations within the electronic image thatare within a neighborhood of the transformed coordinates of the digitalwatermark signal, updating locations of the transformed coordinates tothe peak locations, determining a new geometric transform that maps thedigital watermark signal to the updated locations, and determiningwhether correlation has increased between the electronic image and thedigital watermark signal, transformed by the new geometric transform;the processor further configured to obtain phase estimates of a digitalwatermark signal in the electronic image by estimating phase atnon-integer coordinates corresponding to coordinates of the digitalwatermark signal transformed with the new geometric transform, theestimating employing point spread function parameters to estimate phaseat the non-integer coordinates based on frequency domain data atneighboring integer coordinates; and determining deviation betweenestimated phase and expected phase of the digital watermark signal. 18.The digital watermark detector of claim 17 wherein the candidategeometric transform comprises rotation, differential scale, and shear.19. The digital watermark detector of claim 17 wherein the digitalwatermark signal comprises peaks at the coordinates of the digitalwatermark signal.
 20. A non-transitory, computer readable medium onwhich is stored instructions, which, when executed by a processor,preform a method of determining a geometric transformation of an image,the method comprising: obtaining an electronic image sampled from aphysical object; determining a candidate geometric transform between adigital watermark signal and the electronic image; refining thecandidate geometric transform by transforming coordinates of the digitalwatermark signal with the geometric transform, determining peaklocations within the electronic image that are within a neighborhood ofthe transformed coordinates of the digital watermark signal, updatinglocations of the transformed coordinates to the peak locations,determining a new geometric transform that maps the digital watermarksignal to the updated locations, and determining whether correlation hasincreased between the electronic image and the digital watermark signal,transformed by the new geometric transform; from the electronic image,estimating phase at non-integer coordinates corresponding to coordinatesof the digital watermark signal transformed with the new geometrictransform; and determining deviation between estimated phase andexpected phase of the digital watermark signal.
 21. The computerreadable medium of claim 20 wherein the candidate geometric transformcomprises two or more affine transform parameters.
 22. The computerreadable medium of claim 21 wherein the two or more affine transformparameters comprise rotation, differential scale and shear.
 23. Thecomputer readable medium of claim 20 wherein the digital watermarksignal comprises peaks at the coordinates of the digital watermarksignal.
 24. The computer readable medium of claim 20 further comprising:estimating phase of the digital watermark signal, when transformed bythe geometric transform, by sampling phase from neighboring imagesamples at integer locations within the electronic image signal, andusing the phase estimates of the digital watermark signal to assesswhether the geometric transform improves correlation between theelectronic image and the digital watermark signal, as transformed by thegeometric transform.